Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to C. H. Bennett (the '410 patent), and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. During the QKD process, Alice uses a random number generator (RNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding) and sends this qubit to Bob.
The above mentioned publications by Bennet each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent) discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
There are two main problems with a one-way interferometer used for QKD. One of them involves time variance of the quantum signal polarization. One needs to know the polarization state of the quantum signal precisely as it arrives at Bob. Otherwise, it is very difficult to modulate the signal and keep the interferometer balanced. Another problem involves temperature drift in the interferometer, which causes a phase shift that destroys the intended interference.
Because of these problems and the lack of self-compensation in a one-way system, there is a need for systems and methods relating to stabilizing an interferometer so it can be used as part of a one-way QKD system. This need is particularly acute for manufacturing and selling a commercially viable one-way QKD system.
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